Harold C. Arp, a noted and controversial cosmologist of the twentienth century, is recognized for having catalogued 338 galaxy groups whose galaxy shapes do not conform to one of the two standard types (spiral or elliptical) and their associated subtypes which describe the spiral nature of their swirling arms or elliptical eccentricity. These irregular galaxies are believed to be valuable abnormalities which may provide clues into the ultimate formation of spiral and elliptical galaxies and particularly due to our inability to influence potential factors in galaxy formation when attempting to describe or even comprehend these dynamics.

Each of the 338 entries in Arp's catalog is classified into one of six major types, namely Spiral Galaxies, Spiral Galaxies with Companions on Arms, Elliptical and Elliptical-like Galaxies, Galaxies (not classifiable as Spiral or Elliptical), Group Character (Interacting Galaxies) and Miscellaneous. A second subdivision accompanies each of the initial five major classifications and includes, for example, "One-Armed", "Low Surface Brightness Companions", "Repelling Spiral Arms", "Galaxies with Jets" and "Interacting Galaxies".

Note: Harold Arp's Atlas Of Peculiar Galaxies (1966) is available in its entirety here.